Douglas-fir raises xylem safety in response to a drier climate but also increases supported leaf area

Phenotypic plasticity in traits related to plant water relations and hydraulics is fundamental for the adjustment of trees to rapid climate change. It is not fully understood how conifers can acclimatize their hydraulic system and foliage to a reduction in water availability. For the economically important species Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), we assessed the acclimation potential to a drier climate for mature trees of a common seed source by exploring the phenotypic plasticity of 15 hydraulic and water status-related traits across a steep precipitation gradient in the North German lowlands. Branch embolism resistance (P12, P50), turgor loss point (ΨTLP), hydraulic safety margin (HSM), Huber value, foliage area, needle lifespan and leaf mass δ18O and δ13C were measured. Across the 10 study sites, precipitation explained a large proportion of the variance in P12, P50, ΨTLP, leaf δ18O and δ13C and Huber value, while its influence on foliar traits was small. P12 and P50 became more resistant by ~ 0.2 MPa and ΨTLP decreased by ~ 0.1 MPa with a precipitation reduction by 310 mm year-1, indicating a significant increase in HSM with increasing climatic aridity; however, the extent of adjustment was small. Contrary to expectation, needle lifespan and foliage area increased, while Huber value decreased, with a reduction in precipitation, suggesting greater foliage drought exposure at drier sites. We found fairly high plasticity in hydraulic and foliar traits and enhanced embolism resistance in drier climates, which might distinguish Douglas-fir from other conifers. However, the Huber value reduction with decreasing precipitation suggests drought vulnerability in drier lowland regions.